Coefficients are added together because they are like terms, this can be proven with the distributive property. For example, x(2x+x)=2x^2+x^2=3x^2.
The commutative property of addition and the associative property demonstrate this.
The word "commutative" comes from "commute" or "move around", so the Commutative Property<span> is the one that refers to moving values around. </span> The associative property<span> states that you can add or multiply regardless of how the numbers are grouped. </span>
